三基色測量法在S-GAP推進劑火焰溫度分布中的應用

蔚紅建，付小龍，李吉禎，樊學忠，王 瑛，張國防

(1．西安近代化學研究所，西安 710065;2．陜西師范大學 化學與材料學院，西安 710062)

0 Introduction

The combustion flame characteristics of the propellant are of great value for the theoretical research and application［1－4］，the well-known “Summer-field”combustion model was proposed on the determination of the combustion wave of the propellants［5－6］．Researchers have developed many ways，such as Fourier transform infrared spectroscopy［7］，CARS［8］，thermocouple［9－10］and holographic method［11］etc．，to determine the combustion flame characteristics of the propellant．Currently，the thermocouple method［12］can be used to analyze the flame temperature of the propellant in the pressure of higher than 1 MPa．However，the disadvantages of this method，including limited working temperature range 300～2 300℃，unstable data acquisition in final plateau，volatile in high pressure，disturbing wave structure for its existence in the flame，are too obvious to be applied for precise temperature determination．

In this paper，primary color measurement(PCM)，which is featured in non-contact，instantaneous and wide temperature determination range，was employed for the determination of the temperature distributions of the combustion flame structure of the S-GAP propellants［13］．

1 Experimental

1．1 Materials and preparation of samples

Three S-GAP propellant formulations were designed to determine flame temperature by PCM method，the composition and energetic properties of the propellants are listed in Table 1．

Table 1 Compositions of S-GAP propellants

HMX(Cyclo tetramethylenetetranitramine)is the oxider，NG(nitroglycerine)is the plasticizer，S-GAP is the binder of the propellant，Tcrefers to the combustion temperature of the propellant in rocket motor，Torefers to the flame temperature of the propellant in the atmosphere．

The propellant was prepared in a 5-liter planetary vertical mixer．S-GAP，plasticizers were added and mixed until well wetted，the mixer was vacuumized．After mixing for 60 min，the slurry was casted into block molds under vacuum and cured at 50℃ for 72 h．All the samples were prepared to suitable dimension．

1．2 Apparatus and experimentation

1．2．1 Flame structure analysis method

The thermal video systems(as shown in Fig．1)was introduced to test the flame structure of S-GAP propellants and take a photo of the flame at various pressures．The sample was placed vertically on the ignition rack，and the rack was fixed in a combustion chamber with visual windows．The chamber was filled with continuous flow of nitrogen atmosphere from bottom to top at constant pressure，which can ensure the flame transparency in the chamber．The nickel-chrome wire was used to ignite the propellant samples，in this way the flame pictures were obtained．

Fig．1 Apparatus of thermal video systems with CCD camera

1．2．2 PCM of flame temperature distribution

PCM of flame temperature distribution is based on Plank＇s law，Mie＇s theory and Color coefficient equation．

1．2．2．1 Plank＇s law

The mathematic formula for blackbody radiation is given by Plank［14］．

where Eb(λ，T)is monochromatic radiant intensity，λ is wavelength;T is absolute temperature;C1is Plank first constant;C2is Plank second constant．

1．2．2．2 Mie＇s theory

Mie＇s theory is used to luminous flame radiation by Hottel and Broughto［15］，and the emissivity model of luminous flame is deduced．

where εfis emissivity of luminous flame;k is absorption coefficient;L is geometric thickness;α is coefficient．

1．2．2．3 Color coefficient equation

Color coefficient equation of luminous flame is described by equation(3)［16］．

where R，G，B are color coefficient and theirs integrating range are from 0．38 to 0．78 at visible light．

1．2．2．4 PCM

Spectral radiant power function of luminous flame is described by equation(4)．

PCM was obtained by equation(2)，(3)and(4)．

where k0is system gain;Ki(i=R，G，B)is scale factor;λi(i=R，G，B)is wavelength．

Colorimetric temperature equation was obtained by equation(1)and(5)．

According to known temperature，the liner equation can be obtained．

Therefore，the temperature of PCM is obtained by equation(8)and(9)．

From equation(10)we can see that，when R，G，B are measured，the temperature of luminous flame could be derived．

1．2．2．5 Temperature calibration

By image acquisition，temperature calibration experiment can gain gray level respectively(R，G，B)from standard illuminant A，which is an approximate blackbody of a known temperature．The calibration range is from 1 600 K to 2 400 K．

Table 2 Temperature calibration parameters

The coefficient could be gained by linear fitting．a0=2．323 5 ×10－4，a1=1．122 2 ×10－4，a2=0．697 34 ×10－4．

1．2．3 Combustion wave test method

The Π type double tungsten-rhenium thermocouple was used to test the combustion wave distribution of the solid propellant．The thermocouple(φ =25 μm)was embedded in the propellant sample(diameter=7 mm，length=120 mm)whose profile was coated by polyvinyl alcohol solvent as a flame-retardant and then exposured to air for drying．The nichrome wire(φ =0．15 mm)with directcurrent voltage of 200 V was adopted for ignition．The automatic trigger acquisition system began to record the data output by the thermocouple when the ignition of the propellant．With the sample burning out，the thermocouple approached gradually the burning surface and got into finally the flame zone．In this way，the whole burning process of the propellant was recorded and the combustion wave from the condensed phase to gas phase was obtained ultimately．

2 Results and discussion

2．1 The flame structures of S-GAP propellants

The flame structures of S-GAP propellants at 1 MPa and 3 MPa are shown in Fig．2．

As shown in Fig．2(a)to Fig．2(f)，the flame structure of S-GAP-1 propellant is consisted of three parts，the burning surface，the dark zone and the flame zone．The luminous flame zone of S-GAP-1 is at a distance with the burning surface．The flame zone of S-GAP-1 gradually approaches the burning surface at 3MPa．The dark zone is not obvious．

Similarly，the flame structures of S-GAP-2 and SGAP-3 propellant are consisted of three parts，the burning surface，the dark zone and the flame zone．The luminous flame zones of S-GAP-2 and S-GAP-3 are at some distance off the burning surface．The flame zone of S-GAP-2 gradually approaches the burning surface at 3 MPa．The dark zone is not obvious．

Fig．2 Flame structures of S-GAP propellants at the pressures of 1 MPa and 3 MPa

2．2 The temperature distribution of S-GAP propellants by PCM

The temperature distributions of S-GAP propellants determined by PCM at 1 MPa and 3 MPa are shown in Fig．3．

Interestingly，as shown in Fig．3(a)to Fig．3(b)，the flame temperature distribution of S-GAP-1 is of isothermal．The temperature range of flame zone of S-GAP-1 is between 975．45 ～1 406．24 ℃ at 1 MPa，and the temperatures of flame zone of S-GAP-1 is in therange of 1 266．37 ～1 765．26 ℃ at 3 MPa．It indicate that the temperature increases as the pressure increases by using PCM method．The flame temperature increases gradually from inner to outer．

Similarly，the flame temperature distribution of S-GAP-2 and S-GAP-3 was a series of isotherms．The flame temperature distribution is shown in in Fig．3(c)to Fig．3(f)．

From the results，it is also found that the flame temperatures of S-GAP propellants increases with the increase in the content of HMX in the S-GAP propellants．

Fig．3 Temperature distributions of S-GAP propellants by PCM at 1 MPa and 3 MPa

2．3 The combustion wave structures of S-GAP propellants

The combustion wave structures and temperature distribution of S-GAP propellants are shown in Fig．4 and Table 2．

In Table 2，Tsis the temperature of burning surface;Tdis the temperature of dark zone;Tfis the temperature of flame zone．

In Fig．4(a)and Table 2，the temperature distribution of combustion wave of S-GAP-1 at 1 MPa is deter-mined．The temperature of flame zone is 1 248．24℃．Tfis close to the temperature of the outer flame by PCM．The temperature distribution of combustion wave of S-GAP-1 at 3 MPa is shown in Fig．4(b)and Table 2．The temperature of flame zone is 1 678．83℃．Tfis close to the temperature of the outer flame by using PCM．The tempreature increases as the pressure increases by using combustion wave method．It is probably due to the enhancement of the caloric feedback of buring surface as the pressure increases．

Fig．4 The combustion wave structures of S-GAP propellants at 1 MPa and 3 MPa．

Table 3 Temperature distribution of combustion wave of S-GAP-3 at 1 MPa and 3 MPa

3 Conclusions

(1)PCM is suitable to measure the flame temperature of S-GAP propellant，and the isotherm of S-GAP propellant could be determined by this method．

(2)The flame zone approach the burning surface as pressure increases，the dark area become thinner as the content of HMX in the propellant increased．

(3)The flame temperatures of the S-GAP propellants increase as the pressure increases．

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